Abstract

The prototypic histopathologic criteria including broad cellular septa and/or single hyperchromatic cells, with or without lipoblasts have long been the bane of the diagnosis of atypical lipomatous tumor (ALT). The absence of these findings almost negates the diagnosis of ALT. Current testing guidelines for MDM2 gene amplification by fluorescence in situ hybridization (FISH) includes retroperitoneal location, ‘‘relapsing lipomas,’’ questionable cytologic atypia and large lipomatous tumors (>10 cm) without atypia. We report a perplexing case of superficial borderline well-differentiated adipocytic tumor (SBWDAT), lacking diagnostic histopathologic features of ALT, containing MDM2 gene amplification but with a paradoxical methylation profile of lipoma.

A 66-year-old female who presented with a superficial large (7.4 x 7 x 3 cm) mass in the lateral leg for a year. Histopathology demonstrated a lipomatous tumor with bland cytology that lacked the single hyperchromatic cells or broad cellular septa, and hence was initially diagnosed as lipoma, and further corroborated by an expert soft tissue pathologist. By immunohistochemistry, there were rare, scattered positivity for mdm2, but FISH showed MDM2 amplification. Sarcoma methylation classifier confirmed MDM2 amplification but showed a maximum score of 0.989, consistent with methylation class lipoma and a flat profile in copy number analysis. O[6]-methylguanine-DNA methyltransferase (MGMT) promoter methylation status prediction indicated a score of 0.012 (unmethylated).

We report of unusual case of SBWDAT manifesting a morphologically “silent” (cytologically bland) phenotype with MDM2 gene amplification but a methylation profile of lipoma. Whether this represents a variant in the spectrum of ALT, or a biologically distinct entity is unclear.